1. Field of the Invention
The present invention relates to light scanning apparatuses including a light source that emits a plurality of light beams for exposure of a photosensitive member, and image forming apparatuses including the light scanning apparatus.
2. Description of the Related Art
Conventionally known image forming apparatuses are configured to deflect a light beam emitted from a light source by a rotary polygon mirror and scan a photosensitive member with the light beam deflected by the rotary polygon mirror to form an electrostatic latent image on the photosensitive member. Such an image forming apparatus is provided with an optical sensor to detect a light beam deflected by the rotary polygon mirror. The optical sensor generates a synchronization signal, based on which a light beam is emitted from the light source, thus bringing starting positions of electrostatic latent images (images) into coincidence with each other in the scanning direction (main scanning direction) of the light beam on the photosensitive member.
For a higher image forming speed and higher resolution of images, a known image forming apparatus includes a light source in which a plurality of light emitting elements each emitting a light beam are arranged as shown in FIG. 9A. In FIG. 9A, X-axis direction corresponds to the main scanning direction and Y-axis direction corresponds to the rotational direction (vertical scanning direction) of the photosensitive member. Such an image forming apparatus is adjusted in the assembly process at the factory about an interval between the light emitting elements in Y-axis direction while rotating the light source in the direction of the arrow shown in FIG. 9A. While rotating the light source in this way, an interval between exposure positions on the photosensitive member in the vertical scanning direction of the light beams emitted from the light emitting elements is adjusted to be an interval corresponding to the resolution of the image forming apparatus.
As the light source rotates in the direction of the arrow shown in FIG. 9A, however, an interval between the light emitting elements changes not only in Y-axis direction but also in X-axis direction. Then, a conventional image forming apparatus includes an optical sensor generating a horizontal synchronization signal, based on which each light emitting element is allowed to emit a light beam at a timing specified for the light emitting element, thus bringing the starting positions of the electrostatic latent images into coincidence with each other.
In the aforementioned assembly process, the angle (adjustment amount) to rotate the light source is different for each imaging forming apparatus because the light source may be differently mounted in different image forming apparatuses or optical members such as lenses and mirrors have different optical properties. This means that a plurality of image forming apparatuses have different intervals between light emitting elements in X-axis direction after the rotation adjustment of their light sources. In that case, when the emission timings of light beams from the light emitting elements are uniformly set for all image forming apparatuses based on the synchronization signal generated by the optical sensor, then some of the imaging forming apparatuses may have a starting position of an electrostatic latent image displaced in the main scanning direction.
In order to suppress such displacement of the starting position of an electrostatic latent image in the main scanning direction due to the rotation of the light source in the assembly process, Japanese Patent Application Laid-Open No. 2008-89695 discloses an image forming apparatus including a first light emitting element and a second light emitting element, each of which emits a light beam. A plurality of horizontal synchronization signals are generated based on the light beams emitted, and based on a difference in generation timing between the plurality of horizontal synchronization signals, an emission timing of a light beam from the second light emitting element is set with reference to the emission timing of a light beam from the first light emitting element.
Japanese Patent Application Laid-Open No. 2011-48085 discloses a light scanning apparatus including a lens made of resin as an fθ lens and including an optical sensor to receive a light beam passing through a light-gathering lens (BD lens) different from the fθ lens, thus generating a synchronization signal.
The BD lens of Japanese Patent Application Laid-Open No. 2011-48085 made of resin similarly to the fθ lens leads to the following problem. A BD lens has a property of refracting a light beam in the direction corresponding to the main scanning direction. As the temperature inside the light scanning apparatus increases due to the rotation of the rotary polygon mirror, the property of the BD lens to refract a light beam changes, resulting in the possibility of changing a generating timing of a horizontal synchronization signal. In the case of the image forming apparatus disclosed by Japanese Patent Application Laid-Open No. 2008-89695, detected generating timings of the plurality of horizontal synchronization signals are affected by the change in properties of the BD lens, so that the difference in generation timing between the plurality of horizontal synchronization signals will change and thus accuracy to correct the starting position of an electrostatic latent image will deteriorate.